The present invention relates to the field of transmission via optical fibers and more specifically to compensating chromatic dispersion and chromatic dispersion slope in optical fiber transmission systems.
The index profile of optical fibers is generally described by the shape of the graph of the function that relates the refractive index of the fiber and its radius. It is conventional to plot the distance r from the center of the fiber on the abscissa axis and the difference between the refractive index and the refractive index of the cladding of the fiber on the ordinate axis. The expressions xe2x80x9cstepxe2x80x9d index profile, xe2x80x9ctrapeziumxe2x80x9d index profile and xe2x80x9ctrianglexe2x80x9d index profile are therefore used with reference to graphs that are respectively step-shaped, trapezium-shaped and triangular. These curves are generally representative of the theoretical or set point profile of the fiber and fiber fabrication constraints can yield a significantly different profile.
It advantageous to manage chromatic dispersion in new high bit rate wavelength division multiplexed transmission networks, especially for bit rates greater than or equal to 40 Gbit/s or 160 Gbit/s; the objective, in order to limit pulse widening, is to obtain substantially zero cumulative chromatic dispersion over the link for all wavelengths of the multiplex. A cumulative dispersion value of a few tens of ps/nm is generally acceptable. It is also beneficial to avoid zero values of the local chromatic dispersion, for which the non-linear effects are strongest, in the vicinity of wavelengths used in the system. Furthermore, to prevent or limit distortion between multiplex channels, it is also beneficial to limit the cumulative chromatic dispersion slope over the range of the multiplex. The chromatic dispersion slope is conventionally defined as the derivative of chromatic dispersion with respect to wavelength. Finally, it is also necessary to take account of the fact that the amplitude of non-linear effects in a fiber is inversely proportional to the effective surface area of the fiber. To limit non-linear effects, the effective surface area should therefore ideally be as high as possible. However, some non-linear effects, such as the Raman effect, are useful for improving the margins of the transmission system.
Stepped index fibers, also known as single mode fibers (SMF), are conventionally used as line fibers in optical fiber transmission systems. The applicant""s ASMF 200 stepped index monomode fiber has a chromatic dispersion cancellation wavelength xcex0 from 1300 to 1320 nm and a chromatic dispersion less than or equal to 3.5 ps/(nm.km) in a range from 1285 to 1330 nm and of the order of 17 ps/(nm.km) at 1550 nm. The chromatic dispersion slope at 1550 nm is of the order of 0.06 ps/(nm2.km). This fiber typically has a chromatic dispersion to chromatic dispersion slope ratio C/Cxe2x80x2 from 250 to 370 nm at 1550 nm. This fiber has an effective surface area of around 80 xcexcm2 at 1550 nm.
Dispersion shifted fibers (DSF) have also become available. Non-zero dispersion shifted fibers (NZ-DSF+) are dispersion shifted fibers having a positive non-zero chromatic dispersion at the wavelengths at which they are used, typically around 1550 nm. At these wavelengths these fibers have a low chromatic dispersion, typically less than 11 ps/(nm.km) at 1550 nm, and a chromatic dispersion slope from 0.04 to 0.1 ps/(nm2.km).
The document FR-A-2 790 107 proposes a line fiber especially suitable for dense wavelength division multiplex transmission with a channel spacing of 100 GHz or less and a bit rate per channel of 10 Gbit/s; at a wavelength of 1550 nm, this fiber has an effective surface area greater than or equal to 60 xcexcm2, a chromatic dispersion from 6 to 10 ps/(nm.km), and a chromatic dispersion slope less than 0.07 ps/(nm2.km). At 1550 nm the applicant""s TeraLight fiber typically has a chromatic dispersion of 8 ps/(nm.km) and a chromatic dispersion slope of 0.058 ps/(nm2.km). The ratio of chromatic dispersion to chromatic dispersion slope of this fiber is 140 nm. This fiber has an effective surface area Seff of the order of 65 xcexcm2 and an effective surface area slope of the order of 0.17%/nm. The document FR-A-2 795 828 describes a dispersion compensating fiber suitable for this line fiber.
Using short lengths of dispersion compensating fiber (DCF) to compensate chromatic dispersion and chromatic dispersion slope in SMF or NZ-DSF+ used as line fibers is known in the art. DCF are described in various patents. At a wavelength in the vicinity of 1550 nm they have a negative chromatic dispersion to compensate the cumulative chromatic dispersion in the line fiber, and they can also have a negative chromatic dispersion slope to compensate the positive chromatic dispersion slope of the line fiber. The document U.S. Pat. No. 5,568,583 and the document U.S. Pat. No. 5,361,319 propose a DCF for compensating chromatic dispersion in an SMF which has dispersion of the order of 17 ps/(nm.km) at 1550 nm. Dispersion compensating fibers are generally more costly than line fibers and have a high attenuation.
It is therefore beneficial to have a line fiber that requires as short as possible a length of dispersion compensating fiber; another technical problem is to obtain a fiber enabling transmission over as wide a band as possible.
French patent application 00 02 316 filed Feb. 24, 2000 by the applicant, whose title in translation is xe2x80x9cAn optical fiber exhibiting monomode behavior in-cable for wavelength division multiplex optical fiber transmission networksxe2x80x9d, describes an optical fiber which is used as line fiber and whose chromatic dispersion is compensated by the kind of dispersion compensating fiber conventionally used for a stepped index fiber. At a wavelength of 1550 nm, this fiber has a chromatic dispersion from 5 to 11 ps/(nm.km), a ratio of chromatic dispersion to chromatic dispersion slope from 250 to 370 nm, an effective surface area at least equal to 50 xcexcm2, and a ratio of the square of the effective surface area to the chromatic dispersion slope greater than 80 000 xcexcm4.nm2.km/ps. The above patent application does not refer to the effective surface area slope of the fiber and does not indicate the advantages that such a slope can have.
The invention proposes a fiber that simplifies wavelength management. It can in particular be used as line fiber in wavelength division multiplex transmission systems; the line fiber can have its dispersion compensated by a shorter length of dispersion compensating fiber than a prior art SMF. Also, this fiber is suitable for use over a wide band without significant variations in the transmission properties of the fiber.
To be more precise, the invention proposes an optical fiber having, at a wavelength of 1550 nm, a chromatic dispersion C from 5 to 11 ps/(nm.km), a ratio C/Cxe2x80x2 of chromatic dispersion to chromatic dispersion slope from 250 to 370 nm, a derivative Sxe2x80x2eff of effective surface area with respect to wavelength less than 0.14%/nm, a chromatic dispersion cancellation wavelength xcex0 less than or equal to 1370 nm and an effective surface area greater than or equal to 50 xcexcm2.
The fiber advantageously also has one or more of the following optical characteristics:
it exhibits monomode behavior in-cable in a range of wavelengths from 1460 nm, and preferably from 1300 nm;
it has a theoretical cut-off wavelength less than or equal to 1850 nm and preferably less than or equal to 1800 nm;
it has an effective surface area greater than or equal to 45 xcexcm2 at a wavelength of 1460 nm;
it has curvature losses less than or equal to 400 dB/m at a wavelength of 1625 nm, and preferably at a wavelength of 1675 nm, when wound onto a 10 mm radius former;
it has curvature losses less than 0.5 dB, and preferably less than 5xc3x9710xe2x88x922 dB, at a wavelength of 1625 nm, and preferably at a wavelength of 1675 nm, when 100 turns are wound onto a 30 mm radius former;
it has a polarization modal dispersion less than or equal to 0.1 ps/kmxc2xd, and preferably 0.05 ps/kmxc2xd, at a wavelength of 1550 nm;
it has an attenuation of less than 0.24 dB/km, or even less than 0.22 dB/km, at a wavelength of 1550 nm;
it has a ratio of the square of the effective surface area to the chromatic dispersion slope greater than or equal to 80 000 xcexcm4.nm.km/ps at a wavelength of 1550 nm; and
it has a sensitivity to microcurvatures less than 1, and preferably less than 0.8, at a wavelength of 1550 nm.
One embodiment of the fiber has a trapezium or rectangle with buried trench and ring index profile which advantageously has one or more of the following characteristics:
the difference (xcex94n1) between the index of the trapezium or the rectangle and the index of the cladding is from 5.9xc3x9710xe2x88x923 to 7.8xc3x9710xe2x88x923 and the radius (r1) of the part of the fiber having an index greater than that of the cladding is from 3.5 to 4.5 xcexcm;
the difference (xcex94n2) between the index of the depleted trench and the index of the cladding is from xe2x88x928xc3x9710xe2x88x923 to 1xc3x9710xe2x88x923 and the outside radius (r2) of said trench is from 5.3 to 8.1 xcexcm;
the difference (xcex94n3) between the index of the ring and the index of the cladding is from 1xc3x9710xe2x88x923 to 8xc3x9710xe2x88x923 and the outside radius (r3) of said ring is from 7.2 to 11.1 xcexcm;
the ratio of the inside radius to the outside radius of the trapezium is greater than 0.4;
the ratio of the inside radius to the outside radius of the trapezium is less than 0.95, or even less than 0.90, and preferably less than 0.8;
twice the integral of the product of the radius and the index between a zero radius and the outside radius (r1) of the central part of the fiber having an index greater than that of the cladding is from 76xc3x9710xe2x88x923 to 114xc3x9710xe2x88x923 xcexcm2;
twice the integral of the index between a zero radius and the outside radius (r1) of the central part of the fiber having an index greater than that of the cladding is from 46xc3x9710xe2x88x923 to 56xc3x9710xe2x88x923 xcexcm2;
twice the integral of the product of the radius and the index between the outside radius (r1) of the central part of the fiber having an index higher than that of the cladding and the inside radius (r2) of the ring is from xe2x88x92170xc3x9710xe2x88x923 to xe2x88x9240xc3x9710xe2x88x923 xcexcm2;
twice the integral of the product of the radius and the index between the inside radius (r2) and the outside radius (r3) of the ring is from 72xc3x9710xe2x88x923 to 198xc3x9710xe2x88x923 xcexcm2;
twice the integral of the product of the radius and the index between the outside radius (r1) of the central part of the fiber having an index greater than that of the cladding and the outside radius (r3) of the ring is from xe2x88x9238xc3x9710xe2x88x923 to 90xc3x9710xe2x88x923 xcexcm2.
The invention further proposes a wavelength division multiplex transmission system using the above fiber as line fiber.